From the looks of it it's a simple gradient overly. But really impossible to tell how it's actually structured without fully buying it. Either way, I doubt that the particles themselves are actually vector-based. It's more likely that they were generated as a pixel graphic in some program and are used as a mask on a layer with a gradient.
If you open the file, try selecting some elements and show screenshots including the layers, appearance and swatches panels, it might be possible to work out.
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